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<a name="Expression-Section"></a>
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<p>
Next: <a href="Builtin-Functions.html#Builtin-Functions" accesskey="n" rel="next">Builtin Functions</a>, Previous: <a href="Evaluation.html#Evaluation" accesskey="p" rel="prev">Evaluation</a>, Up: <a href="Expressions.html#Expressions" accesskey="u" rel="up">Expressions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="The-Section-of-an-Expression"></a>
<h4 class="subsection">3.10.8 The Section of an Expression</h4>
<a name="index-expression-sections"></a>
<a name="index-absolute-expressions"></a>
<a name="index-relative-expressions"></a>
<a name="index-absolute-and-relocatable-symbols"></a>
<a name="index-relocatable-and-absolute-symbols"></a>
<a name="index-symbols_002c-relocatable-and-absolute"></a>
<p>Addresses and symbols may be section relative, or absolute.  A section
relative symbol is relocatable.  If you request relocatable output
using the &lsquo;<samp>-r</samp>&rsquo; option, a further link operation may change the
value of a section relative symbol.  On the other hand, an absolute
symbol will retain the same value throughout any further link
operations.
</p>
<p>Some terms in linker expressions are addresses.  This is true of
section relative symbols and for builtin functions that return an
address, such as <code>ADDR</code>, <code>LOADADDR</code>, <code>ORIGIN</code> and
<code>SEGMENT_START</code>.  Other terms are simply numbers, or are builtin
functions that return a non-address value, such as <code>LENGTH</code>.
One complication is that unless you set <code>LD_FEATURE (&quot;SANE_EXPR&quot;)</code>
(see <a href="Miscellaneous-Commands.html#Miscellaneous-Commands">Miscellaneous Commands</a>), numbers and absolute symbols are treated
differently depending on their location, for compatibility with older
versions of <code>ld</code>.  Expressions appearing outside an output
section definition treat all numbers as absolute addresses.
Expressions appearing inside an output section definition treat
absolute symbols as numbers.  If <code>LD_FEATURE (&quot;SANE_EXPR&quot;)</code> is
given, then absolute symbols and numbers are simply treated as numbers
everywhere.
</p>
<p>In the following simple example,
</p>
<div class="smallexample">
<pre class="smallexample">SECTIONS
  {
    . = 0x100;
    __executable_start = 0x100;
    .data :
    {
      . = 0x10;
      __data_start = 0x10;
      *(.data)
    }
    &hellip;
  }
</pre></div>

<p>both <code>.</code> and <code>__executable_start</code> are set to the absolute
address 0x100 in the first two assignments, then both <code>.</code> and
<code>__data_start</code> are set to 0x10 relative to the <code>.data</code>
section in the second two assignments.
</p>
<p>For expressions involving numbers, relative addresses and absolute
addresses, ld follows these rules to evaluate terms:
</p>
<ul>
<li> Unary operations on an absolute address or number, and binary
operations on two absolute addresses or two numbers, or between one
absolute address and a number, apply the operator to the value(s).
</li><li> Unary operations on a relative address, and binary operations on two
relative addresses in the same section or between one relative address
and a number, apply the operator to the offset part of the address(es).
</li><li> Other binary operations, that is, between two relative addresses not
in the same section, or between a relative address and an absolute
address, first convert any non-absolute term to an absolute address
before applying the operator.
</li></ul>

<p>The result section of each sub-expression is as follows:
</p>
<ul>
<li> An operation involving only numbers results in a number.
</li><li> The result of comparisons, &lsquo;<samp>&amp;&amp;</samp>&rsquo; and &lsquo;<samp>||</samp>&rsquo; is also a number.
</li><li> The result of other binary arithmetic and logical operations on two
relative addresses in the same section or two absolute addresses
(after above conversions) is also a number.
</li><li> The result of other operations on relative addresses or one
relative address and a number, is a relative address in the same
section as the relative operand(s).
</li><li> The result of other operations on absolute addresses (after above
conversions) is an absolute address.
</li></ul>

<p>You can use the builtin function <code>ABSOLUTE</code> to force an expression
to be absolute when it would otherwise be relative.  For example, to
create an absolute symbol set to the address of the end of the output
section &lsquo;<samp>.data</samp>&rsquo;:
</p><div class="smallexample">
<pre class="smallexample">SECTIONS
  {
    .data : { *(.data) _edata = ABSOLUTE(.); }
  }
</pre></div>
<p>If &lsquo;<samp>ABSOLUTE</samp>&rsquo; were not used, &lsquo;<samp>_edata</samp>&rsquo; would be relative to the
&lsquo;<samp>.data</samp>&rsquo; section.
</p>
<p>Using <code>LOADADDR</code> also forces an expression absolute, since this
particular builtin function returns an absolute address.
</p>
<hr>
<div class="header">
<p>
Next: <a href="Builtin-Functions.html#Builtin-Functions" accesskey="n" rel="next">Builtin Functions</a>, Previous: <a href="Evaluation.html#Evaluation" accesskey="p" rel="prev">Evaluation</a>, Up: <a href="Expressions.html#Expressions" accesskey="u" rel="up">Expressions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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